The present invention generally relates to gas generators such as used to inflate air bags in an automobile occupant protection system, and more particularly to an improved gas generator having a variable inflation rate output capable of safely achieving a low onset inflation.
The prior art generally discloses inflation systems for deploying an air bag in a motor vehicle which provide a single gas generator in fluid communication with the uninflated air bag. The gas generator is typically triggered by an air bag firing circuit when the sensed vehicle acceleration exceeds a predetermined threshold value, as through the use of an acceleration-responsive inertial switch and an explosive "squib."
Conventional single gas generator inflation systems suffer from the disadvantage that the onset pressurization/inflation rate is generally set to provide an aggressive or rapid initial inflation in order to achieve a particular inflation time even for an occupant positioned relatively close to the air bag. However, an aggressive and uncontrolled onset rate of pressurization becomes problematic in situations where the occupant is out of position. More specifically, the rapid pressurization can cause the air bag to impact against the occupant with enough force to injure the occupant.
In commonly owned U.S. Pat. No. 5,400,487, Gioutsos et al teach an inflation system which overcomes the above-described problems by utilizing a plurality of gas generators which are controllably ignited to provide a variable inflation profile which can be tailored to any given occupant position and for any crash type. While this arrangement dramatically improves the inflation efficiency so as to maximize an air bag's ability to protect an occupant, it does so at significantly higher expense and complexity. More specifically, the multiple gas generators and squibs add considerable cost to the system, while the firing control circuitry requires sophisticated processors capable of accurately timing the various ignition times.
In U.S. Pat. No. 5,009,855, Nilsson discloses a gas generator which positions an insert within the propellant combustion chamber so as to partially separate the combustion chamber into two sections having different volumes. In operation, an igniter generates a flame front which initially ignites a small portion of the propellant as the flame front passes through the first section of the combustion chamber, and then ultimately the remaining propellant as the flame front passes around the insert and reaches the second section of the combustion chamber. Nilsson teaches that the gas volume produced by the first propellant section gently presses an out-of-position occupant into the vehicle seat before the second propellant section rapidly inflates the bag to the maximum volume within the shortest possible time.
While Nilsson discloses a gas generator arrangement which produces a lower onset rate of inflation, the precise rate of onset can only be controlled to a small degree since the same igniter flame front must be used to ignite both propellant sections.
In commonly owned copending U.S. patent application Ser. No. 08/498,852 to DeSautelle et al, filed on Jul. 6, 1995, entitled "Dual Chamber Nonazide Gas Generator," a gas generator arrangement is taught which utilizes two isolated propellant chambers to produce a low onset rate without sacrificing peak inflation pressure or inflation time. The inherently high ignition temperatures of a nonazide propellant charge located in the first chamber produces enough heat energy to conductively ignite the nonazide propellant charge located in the second chamber.
While the dual chamber nonazide gas generator of U.S. Ser. No. 08/498,852 provides a dramatic improvement in performance over known gas generators, a need still exists for providing a variable gas generator which can produce an even greater range of inflation profiles tailored to more specific firing situations such as 10 percentile and/or out of position vehicle occupants. Therefore, a need still exists for a gas generator which can satisfactorily produce variable inflation pressurization with a low onset rate.